Current wireless communication systems utilize several different radio communication standards and operate at many different frequency bands. In this fractured service environment, mobile terminals operating in multiple systems and frequency bands offer a better service coverage than single band and single-system terminals. One example of a multi-band communication terminal is a mobile terminal operating for example at four GSM bands, namely GSM850 (824-894 MHz), GSM900 (880-960 MHz), GSM1800 (1710-1880 MHz), GSM1900 (1850-1990 MHz) and further at three WCDMA bands, namely WCDMA I (1920-2170 MHz), WCDMA II (1850-1990 MHz) and WCDMA V (824-894 MHz).
In order to enable the utilization of multiple frequency bands by a mobile terminal the reception and transmission of the radio signals need to be arranged. An essential element for an operation in multiple frequency bands is an antenna and necessary circuits, such as matching circuits, thereto residing in the mobile terminal. More specifically, a compact multi-band antenna configuration with good performance is needed to realize the communication of mobile terminals with multiple bands. Currently, one approach is that mobile terminals comprise multiple antennas which are dedicated for different frequency bands, e.g. lower and higher frequencies. At the same time as the space for the antennas in the mobile terminal is becoming very limited there is a need to fit more and more antennas inside the terminal, for example to implement mobile antenna diversity.
For example, a prior art multi-band antenna module may include a substrate, first and second coupling elements i.e. antenna elements, and first and second resonant matching circuits. The first coupling element is mounted to the substrate and particularly adapted to couple a first frequency band to a ground plane through a first port. The second coupling element is also mounted to the substrate, and is adapted to couple a second frequency band to a ground plane through a second port. The ground plane may be the same, but is not itself a part of the antenna module. The first resonant matching circuit is coupled to the first port and is disposed on the substrate and has a plurality of components by means of which it is possible to implement a band-pass filter within the first frequency band and to present high impedance at least in the second frequency band. Similarly, the second resonant matching circuit is coupled to the second port and is also disposed on the substrate. The second series matching circuit has a plurality of components that implement a band-pass filter within the second frequency band and to present high impedance at least in the first frequency band.
The outmost challenge is that as the number of required operating frequency bands is increasing, it is very challenging to cover all the required bandwidths with the prior art antenna solutions. The drawback of the prior art antenna solutions according to one approach is that there is a need for a separate antenna element i.e. a coupling element for each of the frequency band. Alternatively, according to another prior art solution one antenna element, typically self-resonant antenna element, can be applied for lower and higher frequencies but in that case the antenna element needs to be big enough in size in order to realize resonances in multiple frequency bands simultaneously. Both of the prior art approaches result a fundamental challenge especially in the area of mobile terminals, because the volume reserved for antennas in the mobile terminal is limited. Furthermore, the prior art implementations force one to isolate the coupling elements from each other so that a coupling between the antenna branches can be prevented. Thus, it can be said that one challenge with the prior art antenna solutions is that antennas, with each encompassing separate frequency bands, causes waste of antenna volume, as the available antenna volume cannot be entirely utilized at any given frequency. Due to the physical limitations of electrically small antennas, it is beneficial to be able to utilize the entire antenna volume especially at the lower frequencies of operation. This way the reachable frequency bandwidth is maximal.